1. Field of the Invention
The present invention relates to methods for fabricating semiconductor circuitry or nano-scale electro-mechanical systems and provides a cost-effective alternative to standard photolithography or electron beam lithography.
2. Discussion of Related Art
Semiconductors and nano-scale electro-mechanical systems have traditionally been produced using processes such as standard photolithography or electron beam lithography, each of which require significant investment in capital equipment and related facilities.
Thin film processing is a cornerstone of modern integrated circuit industry. All integrated circuits consist of successive deposition and removal of thin film materials. Deposition techniques include spin-coating, chemical vapor deposition and atmospheric or low pressures, evaporation, sputtering, reactive ion deposition. Thin film removal methods include, wet chemical etching, plasma reactive and/or physical etching. IC industry furthermore relies extensively on optical and less-so on electron lithography to define patterns in photo resist. In many of these methods, use of high energy charged and neutral particles is inherently required. In reactive ion sputtering, gases are ionized to create plasma the ionic species of which react with a surface to remove atoms. In evaporation, a material is heated by direct resistive heating or by electron-beam bombardment. In sputtering, accelerated ions bombard surfaces to remove materials. In lithography, the high energy electrons impinge on photo-resist to break or strengthen bonds in positive and negative resists respectively. These bonds are successively exposed to chemicals that remove the exposed or unexposed areas for positive and negative photo-resists respectively.
Electron beam lithography (EBL) is currently the state-of-the-art technique for creating patterns with 20-50 nm minimum feature size. However, EBL systems are expensive to procure (millions of dollars) and maintain (>$100,000/year). Furthermore, since they use raster scanning of 0.1-10 nA/cm2 electron beams to expose patterns, they require long exposure times to deposit the 100-1000 μC/cm2 doses needed for developing the EBL photo resists (See, for example, P. Rai-Choudhury, “Handbook of Microlithography, Micromachining, and Microfabrication” Volume 1: Microlithography, SPIE Press Monograph Vol. PM39). This results in a high cost/run of ˜$10,000 for exposing areas as small as 1 mm2.
In contrast, advanced optical lithography techniques employed in the semiconductor industry can result in cost effective exposure of large areas with minimum feature sizes 40-60 nm. However, optical lithography also requires high capital costs, and the cost of exposure tools and specialized masks (millions of dollars) makes optical lithography unsuitable for R&D and low volume production. Hence, e-beam and optical lithography fail to provide cost-effective lithography solutions to enable truly cost-effective micro or nano-scale systems.
The prior art methods for lithography use e-beams or image projection to expose a photo-resist layer modified by exposure to the e-beam or the projected light impinging on the photo-resist from a distant source. The e-beam and image projector are “far-field” sources of photo-resist modifying energy, and so require columns of vacuum which are expensive to create and maintain.
This has spawned other approaches, including the “active mask” approach of Hyde et al in US App number 20060264016, which posits that “greater resolution may be achievable using contact methods, in which a mask is placed in contact with a substrate.” Hyde's lithographic mask includes a patterned energy emitting layer with an active region that emits an energy flux (e.g., light) at a selected level in response to an electrical input, and at least one inactive region that does not emit light in response to the electrical input. Hyde's lithographic method includes generating an energy (e.g., light) flux and exposing a flux sensitive material (e.g., photo resist or a lipid bilayer) to the energy flux to modify the flux sensitive material. Hyde's patterned energy emitting layer can include an array of light emitting diodes or electrodes which are activated or energized in a static or dynamically changeable pattern. Hyde's patterned energy emitting layer thus requires a separate power supply or source for energy to activate the active mask's light emitting diodes (or other energy emitting source) for “flux” emitted from the active mask.
Hyde's description of how to make and use the active mask is somewhat lacking in detail, however, and so persons of skill in these arts will be forced to engage in substantial development work if a practical, working “contact” lithography system (e.g., for semiconductor fabrication) is needed.
There is a need, therefore, for an inexpensive system and method enabling fast and low-cost nano-lithography (20-50 nm minimum feature size) on large areas (1-100 cm2) to realize low-cost nano-scale systems.